glTF Sheen Roughness vs Standard Roughness

In glTF PBR rendering, standard surface roughness and sheen roughness dictate light diffusion at entirely different physical layers and viewing angles. While standard surface roughness governs the sharpness and spread of specular reflections across the underlying base material using a standard microfacet model, sheen roughness specifically controls the scattering behavior of micro-fibers at grazing angles via the KHR_materials_sheen extension. Understanding the mathematical and visual differences between these two parameters is essential for accurately simulating materials ranging from polished metals to soft textiles.

Standard Surface Roughness in Core glTF

Standard surface roughness is a core parameter in the glTF metallic-roughness PBR workflow. It simulates the microscopic irregularities of a surface, determining whether light reflects sharply like a mirror or diffuses broadly into a dull finish.

  • BRDF Model: glTF uses the Trowbridge-Reitz (GGX) microfacet distribution for standard roughness. This distribution models traditional micro-facets, producing specular highlights with characteristic long tails.
  • Angular Response: Reflections controlled by standard roughness are visible across all viewing angles, obeying standard Fresnel equations where reflectivity increases as the surface turns away from the viewer.
  • Material Types: It is designed for solid, smooth, or macroscopic surfaces such as polished metals, plastics, wet stone, or lacquered wood.

Sheen Roughness in the KHR_materials_sheen Extension

Sheen roughness belongs to an optional extension specifically designed to capture the optical characteristics of cloth, velvet, and peach fuzz, where light interacts with millions of microscopic, upright fibers rather than flat surface irregularities.

  • BRDF Model: Sheen employs alternative microfacet models, predominantly the Charlie distribution. This distribution simulates backscattering and high-angle scattering rather than mirror-like specular reflection.
  • Grazing-Angle Emphasis: Sheen roughness specifically dictates how light spreads along the silhouette or grazing angles (rim lighting) of an object. A lower sheen roughness produces a sharp, bright edge highlight, while a higher sheen roughness disperses that glow softly across a wider area of the fabric.
  • Layering Independence: Sheen is computed as an additional layer on top of the base PBR material. Changing the sheen roughness does not alter the underlying base roughness of the surface underneath.

Key Distinctions Summary

  1. Physical Target: Standard roughness simulates microscopic scratches and pits on a solid surface. Sheen roughness simulates light trapped and scattered by protruding micro-fibers.
  2. Highlight Placement: Standard roughness centers specular highlights relative to the light source reflection vector. Sheen roughness concentrates light predominantly around the perimeter and grazing angles relative to the camera.
  3. Distribution Shape: Standard roughness produces classic specular lobes (GGX), while sheen roughness generates softer, velvet-like retro-reflections designed to mimic forward- and back-scattering in cloth.